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Soft images with New UO Orthos

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#1 Guest_**DONOTDELETE**_*

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Posted 03 June 2004 - 04:29 PM

Hi,

I recently took advantage of Helix-Mfg.'s overstock
sale on select UO Classic Orthos and purchased the
7mm and 9mm. The Orange-Tube C8 has been in mothballs
for over 20 years. Views of Jupiter with the 25mm and
40mm stock Kellner's are crisp. When I try the 12mm
Kellner it starts to get soft but it still has a little "snap" when focusing. I figured it was the
eyepiece(12mm) and expected at least the UO 9mm ortho to be
better but it wasn't. No "snap" when focusing. I've checked rough collimation
and it looks pretty good but I'm not sure. One reason I
got the ortho's was to check collimation at higher
powers looking for Airy
Disk and diffraction rings. At higher powers I can see
them but the seeing never seems to be good enough to
check fine collimation. I can see the radial spikes
when I put the star out of focus and from what I've
read that means turbulence above. Does my problem
sound like collimation or could something else be
going on? I've checked without the diagonal and things
are still soft. How often is seeing in the NE good
enough to check/do fine collimation? Will I know when
seeing is real good by not seeing the radial spikes in
the donut? Is my collimation off?
I want to see what my C8 so I can compare
it to a NS11 and decide to buy or not.



Tnx,

Mike in NJ

#2 bierbelly

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Posted 03 June 2004 - 06:48 PM

Saw an interesting quote when surfing the Celestron_AS forum on Yahoo. There are five major problems with getting crisp views from an SCT: collimation, inadequate cooldown, collimation, poor seeing and collimation. Maybe one of these is your problem.

http://perso.club-in...ult/collim.html

#3 Stelios

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Posted 03 June 2004 - 07:20 PM

I recently purchased the
7mm and 9mm. The Orange-Tube C8 has been in mothballs
for over 20 years. Views of Jupiter with the 25mm and
40mm stock Kellner's are crisp. When I try the 12mm
Kellner it starts to get soft but it still has a little "snap" when focusing.
I figured it was the eyepiece(12mm) and expected at least the UO 9mm ortho to be
better but it wasn't. No "snap" when focusing.


This is quite normal with 9mm and will be worse with 7mm. It is not often that your scope can support crisply 222x. As for 285x it will be a VERY exceptional night -- once or twice a year if you are out a LOT.

I've checked rough collimation and it looks pretty good but I'm not
sure. One reason I got the ortho's was to check collimation at higher
powers looking for Airy Disk and diffraction rings. At higher powers I
can see them but the seeing never seems to be good enough to
check fine collimation. I can see the radial spikes
when I put the star out of focus and from what I've
read that means turbulence above. Does my problem
sound like collimation or could something else be
going on?...


It actually sounds to me like insufficient cooldown. Radial spikes usually indicate tube currents. Leave the scope out for at least two hours (yes, TWO hours -- less is possible on some nights, but you want to be sure) after dark, then go and look (or of course during that time you could be using the scope for DSO's and other low-power viewing). I bet the radial spikes are gone. The bad seeing could manifest itself as a dancing image, but NOT as radial spikes.

FWIW I also have never done the 3rd stage of collimation :shrug: (on the couple of great seeing nights I have had, I chose to just observe and enjoy). If you get it to where the (slightly) defocused circles are completely round and centered, you will have enough to be able to compare. If the circles look like ellipses or off-center, then you have collimation issues.

#4 gazerjim

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Posted 04 June 2004 - 04:06 PM

Yep, and to the above advice, I would add, collimation, collimation and cooldown. The 45 minute cooldown period cited in me 9.25 manual must have been a number pulled from midair by a typing clerk. Only if delta T is < about 4 degrees is this true!


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